In optical systems involving the generation and controlled radiation of long or continuous pulses of light, such as spectroscopy, or solar simulation, where high intensity, color correct illumination of sensitive working areas is required, such as in fiber optics illumination devices, it is advantageous to have a light source capable of producing the highest possible light flux density. Products utilized in such applications include short arc inert gas lamps. An existing short arc lamp includes a sealed chamber containing a gas pressurized to several atmospheres, and an opposed anode and cathode defining an arc gap. A window provides for the transmission of the generated light, and a reflector may be positioned surrounding the arc gap.
FIG. 1 shows an existing short arc lamp 100. The short arc lamp 100 includes a window 110, an external filter 120, a retainer ring support 125, a strut 130, a strut support ring 140, a cathode 160, an anode 170, and a ceramic body 180 having a curved surface 182, and a base 190. The window 110 is made of sapphire or quartz. The strut 130, supported by the strut support ring 140, braces the cathode 160. The anode 170 is mounted on the base 190 and is aligned with the cathode 160 along the axis 101. The ceramic body 180 has a parabolic surface 182 acting as a reflector. The ceramic body 180 is mounted on the base 190 at one end. The external filter 120 is mounted near the window 110 outside of the ceramic body 180 of the lamp 100 to reduce the intensity of light beam with certain wavelengths, such as ultra violet light, infra red light, etc. The retainer ring 125 holds the external filter 120.
One of the problems with the existing short arc lamp is the concentrated beam loading over a small clear aperture area on the filter 120. Such concentrated beam loading is likely to cause cracking and coating crazing of the filter 120, which may lead to spectral shifts and light transmission degradation. Moreover, the arc lamp 100 is typically installed within other equipment, such as a projector. Cracking of the filter 120 may damage the equipment within which the lamp 100 is installed. Besides causing property damages, cracking of the filter at high temperature may also cause injuries (e.g., burns, cut, etc.) on the user(s) of the lamp.
One of the existing solutions to the above problem is to place the external filter 120 as close as possible to the lamp window 110 in order to reduce beam loading concentration at the center of the external filter 120. Another existing solution is to put an ultra violet suppression coating on the lamp window 110 in order to reduce the heat and unwanted ozone and ultra-violet light caused by the ultra violet light from the lamp. Furthermore, some existing arc lamps include a hot mirror to reject infra red light, as well as narrow band filters and heat absorbing glass.
However, the above techniques do not solve the problem of beam loading concentration satisfactorily because the area of the light beam concentration on the external filter 120 remains about the same. Such concentration in a small area on the external filter 120 still makes the filter susceptible to cracking and coating crazing. Furthermore, the use of various coating on the lamp window 110 also increases the manufacturing cost of the lamp.